<!-- Enter a brief description/summary of your PR here. What does it fix/what does it change/how was it tested (even manually, if necessary)? -->
Adds the color slider to the tab color picker
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## PR Checklist
* [x] Closes#7948
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [ ] ~Tests added/passed~
* [ ] ~Documentation updated. If checked, please file a pull request on [our docs repo](https://github.com/MicrosoftDocs/terminal) and link it here: #xxx~
* [ ] ~Schema updated.~
* [x] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #7948
<!-- Provide a more detailed description of the PR, other things fixed or any additional comments/features here -->
<!-- Describe how you validated the behavior. Add automated tests wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed
*Not required*
This commit introduces a context menu for Tab and a new item,
"Color...", which will display a color picker.
A flyout menu, containing a custom flyout, is attached to each tab. The
flyout displays a palette of 16 preset colors and includes a color
picker. When the user selects or clears color, an event is fired, which
is intercepted by the tab to which the flyout belongs.
The changing of the color is achieved by putting the selected color in
the resource dictionary of the tab, using well-defined dictionary keys
(e.g. TabViewItemHeaderBackground). Afterwards the visual state of the
tab is toggled, so that the color change is visible immediately.
Custom-colored tabs will be desaturated (somewhat) by alpha blending
them with the tab bar background.
The flyout menu also contains a 'Close' flyout item.
## Validation Steps Performed
I've validated the behavior manually: start the program via the start
menu. Right click on the tab -> Choose a tab color.
The color flyout is going to be shown. Click a color swatch or click
'Select a custom color' to use the color picker. Use the 'Clear the
current color' to remove the custom color.
Closes#2994. References #3327.
